Modelling, Simulation and Data Analysis in Acoustical Problems

Modelling and simulation in acoustics is currently gaining importance. In fact, with the development and improvement of innovative computational techniques and with the growing need for predictive models, an impressive boost has been observed in several research and application areas, such as noise control, indoor acoustics, and industrial applications. This led us to the proposal of a special issue about “Modelling, Simulation and Data Analysis in Acoustical Problems”, as we believe in the importance of these topics in modern acoustics’ studies. In total, 81 papers were submitted and 33 of them were published, with an acceptance rate of 37.5%. According to the number of papers submitted, it can be affirmed that this is a trending topic in the scientific and academic community and this special issue will try to provide a future reference for the research that will be developed in coming years

Standardization of procedures and calculation models for the numerical simulation of acoustics of enclosed spaces

The present work is an exploration of the feasibility and the accuracy of a hybrid FDTD/GA simulation model when applied to broadband acoustic simulations of large-scale environments. The model selected, which has been developed by the Acoustics and Audio Group research team of the University of Edinburgh (UK), combines a wave-based method, a finite-difference time-domain (FDTD) code, with a traditional GA approach, the ray-tracing method. During the investigation, the attention is focussed on the possibilities recently achieved in the wave-based part of the model thanks to the increasing computational power and to new efficient algorithms adopted. The present thesis concerns the method employed to test and assess these new potentials through various application contexts. Moreover, a thorough analysis of the most significant factors affecting the simulation process is carried out with specific assessments of the outcomes. A sample of four large environments is employed in this study, each one of them for specific peculiar reasons: a traditional opera house, a modern theatre, a multipurpose elliptical hall, and a historical university lecture hall. All the simulation process steps and the method adopted to validate the models are reported and described in detail: the 3D modeling phase, the input data, the setting of proper calculation parameters, the time cost, and the acoustic metrics considered time by time in the halls assessed. The thesis allows defining an objective benchmark between the state-of-art of acoustic simulation in medium and large-scale environments and the new advances allowed by recent research in wave-based methods. Finally, the work outlines theoretical and practical remarks on the effectiveness of the hybrid simulation method adopted, enhancing the existing scientific literature with further data and case studies for an increasingly thorough benchmark of computational acoustics techniques

Modeling EMI Resulting from a Signal Via Transition Through Power/Ground Layers

Signal transitioning through layers on vias are very common in multi-layer printed circuit board (PCB) design. For a signal via transitioning through the internal power and ground planes, the return current must switch from one reference plane to another reference plane. The discontinuity of the return current at the via excites the power and ground planes, and results in noise on the power bus that can lead to signal integrity, as well as EMI problems. Numerical methods, such as the finite-difference time-domain (FDTD), Moment of Methods (MoM), and partial element equivalent circuit (PEEC) method, were employed herein to study this problem. The modeled results are supported by measurements. In addition, a common EMI mitigation approach of adding a decoupling capacitor was investigated with the FDTD method

Time domain analysis of switching transient fields in high voltage substations

Switching operations of circuit breakers and disconnect switches generate transient currents propagating along the substation busbars. At the moment of switching, the busbars temporarily acts as antennae radiating transient electromagnetic fields within the substations. The radiated fields may interfere and disrupt normal operations of electronic equipment used within the substation for measurement, control and communication purposes. Hence there is the need to fully characterise the substation electromagnetic environment as early as the design stage of substation planning and operation to ensure safe operations of the electronic equipment. This paper deals with the computation of transient electromagnetic fields due to switching within a high voltage air-insulated substation (AIS) using the finite difference time domain (FDTD) metho

Characterizing Fracture Aperture and Transport Dynamics with Hydrogeophysics: Theoretical and Expérimental Advances

plusieurs km, les fractures peuvent servir de conduits ou de barrières aux écoulements des fluides et jouent un rôle majeur dans divers processus et applications comme l’extraction des eaux souterraines, la migration des contaminants dans les roches fracturées, le stockage souterrain des déchets nucléaires mais aussi la séquestration du dioxyde de carbone et le stockage géothermique. Les propriétés géométriques d’une fracture telles que les variations d’ouverture sur son plan influencent l’écoulement et le transport du fluide. Au sein d’une fracture, la caractérisation statistique des ouvertures est délicate puisque celles-ci présentent souvent une auto-affinitude ; une propriété qui implique des motifs similaires à plusieurs échelles spatiales. Les expériences hydrologiques classiques par leur caractère discret sur le terrain ne fournissent pas d’informations directes sur les variations d’ouvertures. Cette limitation peut en partie être surmontée par l’hydrogéophysique qui combine des méthodes géophysiques avec des expériences hydrologiques. Dans cette étude, nous présentons des avancées expérimentales et théoriques dédiées à l’utilisation du géoradar (GPR) seul et aussi combinées avec des essais push-pull afin de caractériser plus précisément l’ouverture des fractures. Sur le plan théorique, nous avons utilisé des solutions analytiques et développé un cadre de modélisation pour simuler les réflexions GPR dans le milieu fracturé. Les fractures qui le composent sont caracterisées par des ouvertures hétérogènes mais sont intégrées dans une matrice de roche uniforme. En présence de cette hétérogénéité, nous avons dé- montré que les approches classiques qui reposent sur une ouverture de fracture uniforme, conduisent à des estimations fortement biaisées de l’ouverture moyenne. Ce cadre de mo- délisation est important pour l’utilisation du GPR dans un aquifèr fracturé mais convient également à d’autres applications, telles que l’imagerie des fractures dans le béton. Sur le plan expérimental, nous avons réalisé les premières expériences permettant la visualisation de la migration d’un traceur salin lors d’un test push-pull. Les données GPR obtenues ont permis de mesurer la dynamique du panache du traceur et de décrire la nature des fractures. De plus, elles ont mis en évidence des effets de densité qui sont délétères pour les inférences de la dynamique et des processus naturels d’écoulement, comme l’écoulement ambiant. Pour remédier à ces effets, des mesures ont été réalisées en ajoutant de l’éthanol au traceur salin afin d’obtenir une flottabilité neutre tout en conservant la conductivité électrique. La comparaison des résultats pour les deux types de traceurs montrent que l’ajout d’éthanol permet d’éliminer les effets de densité. C’est pourquoi nous suggérons que des traceurs de densité neutre soient utilisés pour les expériences hydrogéophysiques. Enfin, nous introdui- sons un modèle simultané qui peut simuler les expériences push-pull et GPR. Ce modèle pourrait permettre d’inférer les propriétés d’une fracture par les données réels, ce qui serait pertinent en utilisant des méthodes Markov-chain Monte-Carlo. -- Fractures are ubiquitous in the Earth’s crust. From the scale of a few cm to several km, fractures can act as conduits or barriers to fluid flow and play a major role in several processes and applications, including groundwater extraction, contaminant migration in fractured rock, underground storage of disposed nuclear waste and sequestration of carbon dioxide, as well as geothermal heat migration. A fracture’s geometrical properties such as aperture variations along its plane influence fluid flow and transport within. Nevertheless, it is a challenging task to statistically characterize aperture variations of a single fracture because these are often self-affine; a property that implies similar patterns over several spatial scales. Classical hydrological tests are spatially sparse and do not provide direct information about aperture variations. This limitation can partly be overcome by hydrogeophysics, which combines geophysical methods with hydrological experiments. Here, we present experimental and theoretical advances on the use of ground penetrating radar (GPR) alone and also combined with push-pull tests for improved fracture aperture characterization. On the theoretical aspect we used analytical solutions to develop a modeling framework that simulates GPR reflections from fractures with heterogeneous aperture distributions, embedded in a uniform rock matrix. In the presence of aperture heterogeneity in a single fracture, we demonstrate that classical aperture-inference approaches that rely on uniform fracture properties lead to biased estimates of mean aperture. The modeling framework is suitable for GPR use in fractured rock but is also suitable for other applications such as fracture imaging in concrete. On the experimental side, we present the first experiments in which the migration of a saline tracer is imaged during a push-pull test. The GPR data are informative about the dynamics of the tracer plume and the fractures involved in the experiment, but also highlight density effects that decrease our ability to infer natural flow dynamics and processes, such as ambient flow. We address the density issue by introducing a neutrally-buoyant, yet electrically conductive tracer, which consists of ethanol mixed with a saline tracer. A comparison of results from the two types of tracer tests demonstrates that the addition of ethanol diminishes the density effect; we therefore suggest that neutrally buoyant tracers should be used in hydrogeophysics. Finally, we introduce a simultaneous modeling approach that can simulate the combined experiments. The coupled model can be applied within a Markov-chain Monte- Carlo inversion of the data from the combined experiment to infer a fracture’s geometric properties